For many years acid has been used to effectively remove wellbore damage in wells completed in sandstone formations. In a sandstone formation, which for purposes of this disclosure is one which is principally comprised of silica and clay. It may also contain calcareous constituents, the clay components thereof which can interfere with movement of fluid through the formation to, for example, a wellbore. This interference can be due to clay particles which cause the moving fluid to migrate into and plug passages of the formation and severely reduce the formation's permeability. This occurrence is referred to herein as clay damage. Clay damage can also be caused by clay which swells upon contact with foreign liquids.
In high deliverability wells, common practice is to pump acid at rates and pressures lower than fracturing pressures so as to place acid into the matrix of the formation without creating a hydraulic fracture. In order to cover zones of interest as completely as possible, it is also a common practice to use solid materials (such as benzoic acid flakes, rock salt, paraffin flakes, etc.) in conjunction with stages of acid in an effort to treat the entire zone of interest. Effective staging of acid is critical to remove clay damage over extensive intervals with varying permeabilities. Unfortunately, solid materials which are used to divert acid into all intervals can themselves damage the formation when it becomes impossible to remove them.
Low acid pressure injection rates can cause an additional problem. Low injection rates are defined herein to mean an injection rate less than 5 barrels per minute (BPM). When acid is injected at low rates only one perforation in a "cluster" of perforations or zone may take acid. With continued reaction at low injection rates, there is an even greater likelihood that only one perforation or zone will receive acid.
Therefore, what is needed is a method for diverting acid at high injection rates into all intervals of a formation via a diverting agent which diverting agent does not itself cause additional damage to the formation.